The Genetics of SMA – What is the SMN1 Gene?
Spinal muscular atrophy is caused when an individual has two missing or faulty SMN1 genes. But what is the SMN1 gene, and what does it do?
About the SMN1 Gene
SMN1 stands for survival motor gene 1. This gene is responsible for making a protein called survival motor neuron protein. This protein is found throughout the body. But, as the name might suggest, it is particularly important for the motor neurons cells that live in the spinal cord. The job of motor neurons is to help control muscle movement, and survival motor neuron protein helps the neurons do that job correctly.
Like many genes, we typically inherit one copy of SMN1 from each of our parents.
Though most people have two copies of this gene, the body can function with just one copy. People who have one functioning copy of SMN1 and one missing or faulty copy are called carriers. They don’t have symptoms of SMA, but they carry a faulty gene and could pass that on to their children.
If two carriers have a child, and both pass on the missing or faulty copy, the child will have SMA. The probability of this occurring is 25% for each pregnancy.
About the SMN2 Gene
The human body has a built-in back-up that helps make up for the missing SMN1 gene. This is SMN2, or survival motor neuron gene 2. SMN2 has a splicing error, which means it makes less functional SMN protein.
The number of copies of SMN2 varies from person to person. Often, though not always, individuals who have more copies of SMN2 will have a less severe form of SMA.
One way to treat SMA is to develop drugs that target SMN2, causing it to make higher amounts of function SMN protein. Spinraza, the only FDA-approved treatment for SMA, works in this way.
For Additional Information
To learn more about how these genes were identified, read our recent post about the discovery of SMA.